Gerald L. Gottlieb

Dependence of human ankle compliance on joint angle

Abstract The compliance of the normal human ankle joint has been measured at different mean joint angles. This was done by applying band-limited Gaussian torques to rotate the ankle about a horizontal axis through the medial malleolus. The dependence of the measured compliance on mean joint angle is found to be primarily due to passive properties of the joint and limb tissues. This dependence is small compared to the dependence of joint compliance on the level of muscle contraction. Restrictions on the interpretation of compliance measures bacause of nonlinear and time-varying properties of the motor system are discussed.

Oscillation of the human ankle joint in response to applied sinusoidal torque on the foot

1. Low‐frequency (3‐30 Hz) oscillatory rotation of the ankle joint in plantarflexion—dorsiflexion was generated with a torque motor. Torque, rotation about the ankle and electromyograms (e.m.g.s) for the gastrocnemius—soleus and the anterior tibial muscles were recorded.

Short-term effects of intrathecal baclofen in spasticity

Six patients with long-lasting spasticity resistant to different drug therapies including oral baclofen received a bolus injection of lumbar intrathecal baclofen. Electromyographic (EMG) reactions of leg muscles (soleus, tibialis anterior, quadriceps, and hamstrings) to standard stimuli and during attempts at voluntary activation were recorded before the drug injection and up to 3 h after the injection. Responses to joint movements, H-reflexes, ankle clonus, and defensive reactions were noticeably suppressed within 30-45 min after the injection and had practically disappeared after 2 h. Ankle clonus was seen only in patients with H-reflexes, and clonus disappeared when the reflex responses to the n. tibialis stimuli were absent. A decrease in clonus EMG burst amplitudes was accompanied by a decrease in the clonus frequency. These observations favor the autooscillation hypothesis of clonus. Baclofen injection led to improvement in selective voluntary activation of leg muscles in patients with residual motor control. These results suggest that execution of voluntary motor commands in the patients suffered from functionally abnormal spinal circuitry rather than from changes in the descending motor commands. Intrathecal baclofen appears to be an effective way of eliminating increased muscle tone and spasms which can allow for voluntary motor function when it is present.

Stretch and Hoffmann reflexes during phasic voluntary contractions of the human soleus muscle

Measurements were made of EMG responses in human soleus muscle, following torque disturbances of the foot during phasic voluntary plantarflexions. These were compared with measurements of the Hoffmann reflex during identical voluntary movements. We observe that stretch adequate to evoke a stretch reflex in the relaxed limb often fails to evoke any response during a phasic contraction until about 120 msec after the onset of the stimulus. During this period the H-reflex is facilitated. These findings are discussed in terms of the servo-theory of stretch reflex function and the concept of trans-cortical stretch reflexes.

Extinction of the Hoffman reflex by antidromic conduction

We have shown that one cause of the extinction of the Hoffmann reflex by antidromic conduction in the alpha motor axons must be a relative refractoriness of the motoneuron pool. A hypothesized collision between the H-wave and the antidromic M-wave component is not disproved but it (and the incumbent assumptions about relative afferent and efferent conduction velocities) is shown to be unnecessary. A montonic variation of the M-wave with tonic soleus contraction (M-wave increasing with contraction) is observed but not explained.

Stretch Receptor Models I - Single-Efferent Single-Afferent Innervation

Two lumped parameter models of muscle stretch receptors are described quantitatively on the basis of experimental data from crustacean, amphibian, and mammalian muscle spindles. These models have single-efferent (input) and single-afferent (output) innervation. It is shown that these simple mechanical models can account for many of the behavioral characteristics exhibited by muscle spindle, but are inadequate for a complete model. These inadequacies are discussed. The two models are shown to be sufficiently similar in their transient and steady-state responses to be physiologically equivalent from a systems viewpoint.

Effect of virration on the ankle stretch reflex in man

Vibration at frequencies above 50 Hz applied to the tendon of the extensor muscles of the ankle joint produce the tonic vibration reflex (TVR) which increases when the vibration frequency is increased. The TVR affects a joints mechanical response to sinusoidal and random oscillations in a manner similar to that seen with tonic voluntary contraction. Although the myotatic reflex is suppressed by vibration, repeated stretches of sinusoidal oscillation produce an average EMG response which is not different in magnitude from the no vibration case. Either polysynaptic mechanisms at the spinal cord level of mechanisms involving higher centers (and possibly both) are able to overcome the inhibitory mechanisms at the Ia-alpha motoneuron level in producing a stretch evoked resonance near 6 Hz. The degree of inhibition of the myotatic component of the stretch reflex is proportional to the vibration frequency. This is in contrast to the facilitation of the myotatic reflex produced by tonic voluntary contraction. Vibration does not seem to influence the post-myotatic component (> 100 msec) of the stretch reflex. These results indicate that the post-myotatic responses to limb perturbation are not only different in their latency but also in their functional dependence upon peripheral influences.

Analysis of Step Tracking in Normal Human Subjects

In this communication, some observations of the step-tracking situation in normal human subjects are presented. The muscle systems around the ankle joint were studied. The experiments indicate that the predominant control law seems to be a unilateral activation of the appropriate muscle with some lead compensation. The antagonist muscle is simply turned off and not used as an active brake on the action, with one exception as noted in the results.

Regulation of reflex gain in the control of movement

The myotatic reflex, the simplest and fastest feedback loop of the motor system has not been demonstrated to possess a clear and simple physiological role in motor control. In this paper, we discuss two possible roles, load compensation and sensory-motor integration. Integral to both these roles is the idea that the gain of the closed-loop reflex arc is one of the regulated parameters of the system. Data supporting these roles and the hypothesis of gain control is presented and discussed.

Reflex response to sinusoidal forces at the ankle joint

The dynamic properties of muscle contraction and its neurological control mechanisms have been extensively studied by the application of sinusoidal length changes. Sinusoidal oscillations are easy to generate and the methods of analysis are well understood. Most studies have been in animals which have the advantage of allowing subsystem analysis and recording from points within the system (such as the alpha motoneurons), but these studies do not provide information concerning normal working processes, nor are they in humans. Joyce, Rack & Ross ( 1 9 7 4 ) studied the forced oscillation of the human elbow joint. Many of their results are confirmed by those to be reported here. Walsh (1973) applied sinusoidally varying forces to the wrist and observed a resonance and jump phenomenon. In this paper we will consider the effects of low frequency (3 to 30 Hz) forced sinusoidal oscillation of the ankle joint in normal human subjects.